The present invention relates to a control system for air conditioners installed on vehicles and the like.
Japanese patent application Laid-Open No. 58-141912 discloses an air-conditioner control system which is provided with a microcomputer. Analog temperature data from an indoor-air temperature setter, an indoor-air temperature sensor, an outdoor-air temperature sensor, a sunshine sensor and the like, in other words, voltages corresponding respectively to resistance values of these setter and sensors are inputted, independently of each other, successively to an A/D (analog/digital) converter through a multiplexor. The A/D converter converts the analog temperature data or voltages to digital data which are then inputted successively to the microcomputer. On the basis of these temperature data, the microcomputer executes calculation to send a control signal to an actuator for an air-mixing door, for example, which determines a mixing ratio between hot air and cold air, thereby controlling an opening angle of the air-mixing door.
The arrangement of the control system of the type referred to above is generally such that, on the basis of the temperature data from the indoor-air temperature setter and the plurality of temperature sensors, the microcomputer calculates a single basic temperature value which forms the basis of control. On the basis of the basis temperature value, the computer outputs the control signal to the actuator.
Since, in the above control system, the basic temperature value is calculated by the microcomputer, it is required for the microcomputer to have a high arithmetic capacity. The microcomputer high in arithmetic capacity is large in size and is expensive.
On the other hand, disclosed on pages 21-22 of the service manual entitled "MAZDA (DIESEL KIKI) PURE AUTO-AIR-CONDITIONER SERVICE MANUAL" (CAT. NO. 525179; April, 1984) is an air-conditioner control system which is provided with an operational amplifier. In place of a microcomputer, the operational amplifier calculates an analog basic temperature value on the basis of the indoor-air temperature setter and the various temperature sensors. The operational amplifier has an output terminal and an inverting input terminal which are connected to each other through a negative feedback resistor. The indoor-air temperature setter and the various temperature sensors are connected to the inverting input terminal of the operational amplifier through respective input resistors. A reference voltage from a reference-voltage generating circuit is inputted to a noninverting input terminal of the operational amplifier. On the basis of voltages corresponding respectively to the resistance values of the indoor-air temperature setter and the temperature sensors, the operational amplifier outputs voltage which represents the aforementioned basic temperature value. The output voltage from the operational amplifier is digitized by and A/D converter, and the digitized data are inputted to a microcomputer. With the arrangement disclosed in the above-mentioned service manual, a burden of the microcomputer is lightened correspondingly to the calculation executed by the operational amplifier, so that it is not required for the microcomputer to have very so high arithmetic capacity. Thus, it is possible for the arrangement to use the microcomputer which is small in size and is inexpensive.
In the control system disclosed in the above-mentioned service manual, the circuit for generating the reference voltage inputted to the operational amplifier is composed of a pair of resistors connected in series to each other. Voltage at the junction between the pair of resistors is offered as the reference voltage. Thus, the reference voltage is constant.
By the way, the various resistors employed in the above control system have their respective errors in resistance value. However, these errors cannot be taken up because the reference voltage is constant, so that air cannot be conditioned with high accuracy on the basis of the temperature data from the indoor-air temperature setter and the temperature sensors.
Generally, it is known well to adjust the reference voltage inputted to the operational amplifier, by the use of a variable resistor. It may be considered that an attempt is made to apply this adjusting technique to the above-described air-conditioner control system to take up the aforementioned errors. In such case, however, problems arise as to simplification of the adjusting operation carried out prior to shipment of the control system and simplification of instruments required for the adjusting operation. This is because it is expected that an adjusting operator connects a voltmeter to the output terminal of the operational amplifier and adjusts the reference voltage while watching the voltmeter. The need of the voltmeter, the need of the operation connecting the voltmeter to the operational amplifier, and so on result in an increase in the cost required for the adjustment. In particular, when a finished or completed control system, in which the entire components have been accommodated in a control box, is re-adjusted, the aforesaid adjusting operation is made more troublesome and cumbersome, because the control box is required to be opened.